Hello,
About 3-4 years ago I bought an Olympus E-410, but I find the camera really boring, despite trying to develop my skills in photography I just felt like I was jumping in too quick.
Now I want to replace it, and I'm looking at the following:

Nikon D3100
Olympus E-P3
Sony NEX5

The reason I want the Nikon because I want to rekindle my interest in photography, whilst not getting completely lost (e.g. the guide mode) but I also was looking at the ILC because the E-P3 looks like it's much easier to use and offers interesting photo filters etc.
The NEX5 is also another consideration, but purely an aesthetic one because I think it looks nice.. haha

That's the thing, I'm just a beginner photographer so I don't really specialise in any specifics. I have also been looking at the Pentax K-r, and it does appeal to me more due to the built in stabalisation (not to mention a slightly cheaper price tag). However the only drawback is that it only records 720p- can someone enlighten me if its that big a difference?

In terms of handling the cameras, I managed to play with the NEX5 and P3, and they were ok, although slightly slip and slidey out of my hands. I haven't handled the Nikon, but SLR's seem to be pretty much fine for me because I have big hands so I can handle them with reasonable dexterity.

The 720p VS 1080p debate isn't black and white as resolution isn't an necessarily indication of video quality i.e. 720p video recorded on a DSLR will probably be better quality than 1080p video recorded on a camera phone. However, with like for like video and audio bitrates and frame, 1080p is "better" though almost certainly at the cost of a much larger file size.

The mirrorless bodies may look better aesthetically, but have you ever held them or used them in your own hands? Their grips just aren't as pronounced as those of DSLRs, and I find them very difficult to hold. That's a personal thing though, you'll need to hold each camera out in your hands to determine which one has the better ergonomics in your opinion.

If you want a small body and a small lens combo that will give you great image quality, the E-P3 would probably be the best option. You will have to sacrifice a bit of sensor size to keep the camera and lens' size down, though.

For a small body with a larger APS-C sized sensor in it, the NEX-5 is a better option. Its sensor is probably the best out of the three, however because of its large sensor it has very large lenses in relation to the body, and I find it to be very front heavy. That is of course, once again, a personal thing and you'll need to try the camera out to see if this bothers you.

The D3100 also offers an APS-C sized sensor like the NEX-5, but it's a considerably larger camera. It will feel stabler in your hands, but of course, it's much less compact. However, one major advantage that it offers is a MUCH wider lens selection, plus some 3rd party options to add to your collection.